1. Field of the Invention
The present invention relates to an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus having an improved structure in which a change in the speed of a photoreceptor web can be maintained to a minimum when an image formed on the photoreceptor web is indirectly transferred to a sheet of paper transferring between the fusing roller and the transfer roller.
2. Description of the Related Art
In general, an indirect transfer type image forming apparatus such as a color laser printer or a photocopier is for printing a desired image by developing an electrostatic latent image formed on a photoreceptor medium such as a photoreceptor web or a photoreceptor drum with a developer having a predetermined color and transferring the developed image to a sheet of paper.
Referring to FIG. 1, in the indirect transfer type image forming apparatus, an electrostatic latent image is formed on a surface of a photoreceptor web 10 circulating along an endless path, by an electrostatic latent image forming device such as a laser scanning unit (LSU) 32. A toner image is formed on the surface of the photoreceptor web 10 by developing the electrostatic latent image with a development unit 20. The image forming apparatus includes a transfer unit 30 having a transfer roller 22 and a fusing roller 24 to transfer the toner image formed on the photoreceptor web 10 to a sheet of paper P. However, when a leading edge El of the paper enters a transfer nip Nt between the transfer roller 22 and the fusing roller 24, a pressing force applied to the transfer nip Nt changes due to the thickness of the paper. As a result, the proceeding speed of the photoreceptor web 10 changes.
FIG. 2 is a graph showing changes in the speed of the photoreceptor web 10 at the moment when the leading edge El of paper enters the transfer nip Nt or a trailing edge Et thereof is exhausted from the transfer nip Nt in a conventional indirect transfer type image forming apparatus, and a banding phenomenon of an image according to the above change, obtained through experiments. In the graph, the vertical axis indicates changes (xcex94v) in the speed of the photoreceptor web with respect to a regular speed (v) thereof which is calculated in percent. A positive value means acceleration and a negative value means deceleration in the speed of photoreceptor web 10. The horizontal axis indicates time. Here, a portion A is a moment when the leading edge El enters the transfer nip Nt and a portion B is a moment when a trailing edge Et of the paper is exhausted from the transfer nip Nt. That is, the speed of the photoreceptor web 10 changes due to impact generated when the paper P enters and is exhausted from the transfer nip Nt. The factor possibly ill-affecting the constancy of the photoreceptor web driven by a driving roller is the thickness of the paper itself periodically entering between the transfer roller and the fusing roller, and this actually causes a considerable speed change of the photoreceptor web. The lower portion of the drawing shows an example of an image printed on a sheet of paper effected by changes in the speed of the photoreceptor web. Here, the image at a portion where the speed of the photoreceptor web increases excessively appears light while the image at the portion where the speed of the photoreceptor web decreases excessively appears dark, which is preferred to as image banding.
Here, the moment when the leading edge of the paper 26 enters between the transfer roller 22 and the fusing roller 24 is xcex94t1, the time for maintaining a constant speed after the paper enters is xcex94t2, and the moment when the trailing edge of the paper is exhausted between the transfer roller 22 and the fusing roller 24 is xcex94t3. The type of banding generated to the image, that is, the change in width xcex94L of banding or concentration of the image is closely related to the change in speed of the photoreceptor web 10.
The change of the concentration of the image depends on the amount of the speed change of the photoreceptor web and the width of banding xcex94L depends on the duration of the change of the speed xcex94t. The width of the banding xcex94L can be expressed as shown in Equation 1.
xe2x80x83xcex94L="ugr"xc3x97xcex94txe2x80x83xe2x80x83[Equation 1]
Here "ugr" is the speed of the photoreceptor web.
As the paper passing between the fusing roller and the transfer roller is always pressed by a constant pressing force, at the moment A when the leading edge of the paper is caught between the transfer nip or at the moment B when the trailing edge of the paper is exhausted from the transfer nip, the rotation speed of the transfer roller instantly increases or decreases so that the circulating speed of the photoreceptor web changes considerably. Then, when the electrostatic latent image is formed on the photoreceptor web by the laser scanning unit, a change in the scanning position is generated according to the change of speed of the photoreceptor web as shown in portions A and B of FIG. 2 so that the quality of print is lowered.
To solve the above problems, it is an objective of the present invention to provide a method of reducing banding in an image due to changes in the speed of a photoreceptor medium of an indirect transfer type image forming apparatus, by which the change of speed of the photoreceptor web is minimized during the formation of an electrostatic latent image on the photoreceptor medium using an LSU, when the paper arrives at or is exhausted from the transfer nip, so that a superior print quality can be guaranteed.
Accordingly, to achieve the above objective, there is provided an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus for preventing banding occurring by a change in the speed of the photoreceptor medium generated when the leading edge and trailing edge of a sheet of paper pass through a transfer nip formed between a transfer roller and a fusing roller, which is achieved by (A) separating the transfer roller and the fusing roller a predetermined distance from each other before the leading edge of paper enters the transfer nip, (B) approximating the transfer roller and the fusing roller after the leading edge of paper has entered the transfer nip, and maintaining the transfer nip in a normal state, and (C) separating again the transfer roller and the fusing roller a predetermined distance from each other before the trailing edge of paper is exhausted from the transfer nip.
Also, to achieve the above objective, there is provided an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus for preventing banding occurring by a change in the speed of the photoreceptor medium generated when the leading edge and trailing edge of a sheet of paper pass through a transfer nip formed between a transfer roller and a fusing roller, which is achieved by (A) reducing a pressing force applied between the transfer roller and the fusing roller to be less than the pressing force in a normal state, before the leading edge of paper enters the transfer nip, (B) restoring the pressing force applied between the transfer roller and the fusing roller to the normal state after the leading edge of paper has entered the transfer nip, and (C) reducing the pressing force applied between the transfer roller and the fusing roller less than the pressing force in the normal state, before the trailing edge of paper is exhausted from the transfer nip.